This invention generally relates to methods and apparatus for ink jet printing and plotting but more particularly this invention relates to the field of high resolution ink jet color printing and plotting.
During the last decade the use of electrically controllable ink jets in recording devices such as industrial markers, printers, and color hardcopy output devices for computers, has increased measurably. Two principally different methods have been developed: drop on demand methods and methods using continuous ink jets. Both methods have been used for the printing of alphanumeric characters and to generate computer generated charts and images, often generated in color. Also ink jets have been applied in the field of facsimile transmission.
As a result of intensive research on methods for ink jet printing, the quality of the print out has been improved appreciably during the last few years. The fact of improved quality of print out is especially true for computer controlled ink jet plotters generating color pictures on paper or transparencies, where the ultimate goal is to have an image quality essentially equal to the image quality of a good photographic color print. However, the effort toward achieving this ultimate goal is limited due to the fact that the drop-on-demand method and most of the continuous jet methods are essentially digital devices, i.e., in each picture element (pixel) of the picture they place a drop of ink or no ink at all.
These plotters usually use only four colors of ink (magenta, yellow, cyan, and black). Consequently only a very limited range of color shades can be printed by different combinations of these colors in each pixel. This limitation has been circumvented by the so called dither techniques described by Jarvis, Judice and Ninke in xe2x80x9cComputer Graphics and Image Processingxe2x80x9d, No. 5, 1976, pp 13-40. In the Jarvis, et al article, the picture is divided into a large number of square matrices, each matrix containing a certain number of pixels. A typical matrix size is 4xc3x974 or 8xc3x978 pixels, i.e., 4 pixels in each row and 4 pixels in each column. Thus each pixel has an area of {fraction (1/16)} the area of the matrix. Different shades of color can be obtained by filling different numbers of pixels in each matrix with ink. Hence, using a 4xc3x974 matrix 16 different shades of color and white (no color) can be generated, while an 8xc3x978 matrix allows the rendition of 64 shades of color in addition to white. There has been described several ways of generating color shades by the matrix method above, e.g. ordered dither, digital half tone, and special unordered dither algorithms. However, due to the relatively large size of the ink drops, all of these methods result in coarse images, images so coarse that the eye can clearly discern the repetitive pattern of the matrices or at the least, the eye perceives a strong and very apparent graininess in the generated color print.
The image quality of the matrix method can be improved appreciably if the color density in each pixel can be varied continuously. If the pixels which make up the matrix are smaller than 0.1xc3x970.1 millimeters, then the unaided human eye can no longer resolve the pixels at a normal viewing distance of 20 centimeters. Therefore, an image made up by 8 to 10 of such continuous density pixels to the millimeter has the same appearance, when viewed by the unaided eye, as a truly continuous tone picture, typified by a high grade photographic color print. The fact, of pixels smaller than 0.1xc3x970.1 mm appearing as continuous, is used in conventional high quality color printing using offset or gravure plates, where pixel densities of 6 to 8 pixels per millimeter (160 to 200 per inch) are used and these methods generate color shades by varying the size of each pixel point, i.e., the amount of ink applied to each pixel.
The primary object of the present invention is to essentially apply the method used in offset and gravure printing to ink jet printing resulting in dramatic improvements in image quality and resolution of ink jet color prints.
The central idea of the invention is to control the amount of ink laid down in each pixel by the ink jet. This control of the ink can be attained by using continuous ink jets modulated electrically as described by Hertz in the U.S. Pat. No. 3,916,421. Depending on an electrical signal voltage applied between the ink and a control electrode surrounding the point of drop formation, the drops generated by the continuous jet are either charged or uncharged. When these drops subsequently travel through an electric field, the charged drops are deflected into a catcher while the uncharged drops travel undeflected onto the recording paper.
Now, if the jet diameter is small, e.g., about 10 xcexcm and its velocity is about 40 meters per second, the drops are so small that the mark produced by a single ink drop on the paper cannot be detected by the unaided eye. Therefore, in order to print a pixel in the matrix color method described above, about 30 drops of ink have to be deposited in each pixel. This is achieved by keeping the electrical control signal at 0 volts during the generation of 30 drops, so that their electrical charge is zero which allows them to travel to the paper undeflected. Throughout the remaining portion of this disclosure this electric signal may be called the print pulse. Obviously, if the number of drops generated per second is constant, the length of time duration tp of the print pulse will determine the number of drops in the form of a drop train, travelling towards the paper.
If, e.g., 30 drops of ink are required to generate maximum color density in a pixel, lighter shades of that color can be produced by laying down a smaller number of drops in that pixel. Actually, if it can be assumed that one could determine exactly the number of drops laid down in the pixel by controlling the length of the electrical print pulse, then in this way 30 shades of a color in addition to white (no color) could be generated in each pixel. By the above method then, the color saturation in each pixel can be varied essentially continuously as in the case of conventional offset of gravure printing.
While the principle of the present invention appears to be and in fact is relatively simple, it is actually very difficult to put into practice. Firstly, the diameter of the drops has to be very small and the drop generation rate must be relatively high and essentially constant. Secondly, because of the high air resistance encountered by the very small drops, not only the individual drops but also groups of drops tend to merge on their way to the paper which gives rise to an appreciable graininess especially in the instances of lighter shades of a color. Finally, drops with undefined charges should be avoided for the same reason, that is, merging will take place as noted above.
It is therefore an object of the present invention to control the number of drops deposited in each pixel. According to the invention this objective can be achieved by realizing at least one or more of the following features or combinations of any or all of the following functions or features:
a. the jet diameter should be small its velocity high to ensure a high drop generation rate;
b. ultrasonic stimulation of the drop formation process of the jet should be used to insure a high and constant drop generation rate and to decrease the drop diameter and also providing for or causing the generation of all drops having diameters which are essentially equal to each other, thereby decreasing the tendency for the drops to merge because equal size drops will encounter equal air resistance resulting in essentially equal transit time from nozzle to paper;
c. providing for slightly charging the drops, which are meant to reach the paper, which like charges thereby causing a repulsive force between them which further counteracts the merging tendency of the drops or of the groups of drops;
d. obviously the length of the electrical print pulse determining the number of drops that reach the recording medium, generally paper, can vary continuously. This also tends to cause graininess of the image, which can be avoided by the use of suitable electronic circuitry which adjusts the length of the signal such that it equals n/f, where f is the drop formation frequency and n an integer chosen such that the ratio n/f is close to the length of the original signal and additionally, the start of the print pulse can be synchronized with a suitable phase of the ultrasonic stimulation mentioned under b;
e. providing for an air flow directed along the axis of the jet from the nozzle towards the recording paper to decrease or essentially eliminate the air resistance encountered by the drops; and
f. evacuating, at least partially, the space close to the jet also decreases the air resistance and further decreases the graininess of the picture.
Many other advantages, features, and additional objects of the present invention will become manifest to those versed in the art upon making reference to the detailed description and the accompanying sheet of drawings in which a preferred embodiment incorporating the principles of the present invention is shown by way of illustrative example.